1. Field of the Invention
The present invention relates to an image processing apparatus such as a laser beam printer.
2. Related Background Art
Digital image signals have been frequently used in recent years due to easy processing and transmission. Dither methods and density pattern methods have been proposed to digitize gray scale signals of a halftone image in conventional image processing apparatuses. Using these methods, however, sufficient gradation precision cannot be obtained if a small threshold matrix is used. In order to solve this problem, a large matrix must be used. In this case, however, it is impossible to reproduce images of high quality since image resolution is degraded and a texture structure stands out due to the matrix periodic structure.
A PWM (Pulse Width Modulation) system has been proposed to output a halftone image of high quality and a character or line image of high quality. In this system, an analog (gray scale) video signal is compared with, e.g., triangular pulses synchronized with the video signal to binarize the video signal. According to this scheme, if the binarized video signal after PWM modulation is not accurately transmitted to a laser generation apparatus, a halftone image cannot be reproduced with high quality.
FIG. 2 shows a conventional case wherein inaccurate transmission of the binary signal to the laser generation apparatus is caused by a horizontal sync signal generator circuit. More specifically, if a detection signal is obtained upon radiation of a laser beam onto a beam detector 5, beam pattern data applied to the beam detector 5 is preferably FF.sub.(H) (where H is the hexadecimal notation). On the other hand, if digital data is binarized by a binarizing circuit 1 and the resultant binary signal is input to, e.g., a semiconductor laser 4, an insertion circuit 3 (i.e., an OR gate in this example) (intervention means) is required to insert the beam pattern data FF.sub.(H) in the binary data. When a special circuit, such as the insertion circuit 3 is additionally arranged, circuit parameters of the insertion circuit 3 particularly influence PWM short pulses to slightly change the ON/OFF timings the pulses in the PWM binarizing operation by the binarizing circuit 1. In this case, the image tends not to be accurately reproduced. This problem occurs in not only the PWM binarizing operation but also in the dither binarizing operation.